1. Field of the Invention
The invention relates generally to the production of consolidated cellulosic articles. More particularly, the invention relates to the production of a consolidated cellulosic article having increased thickness
2. Brief Description of Related Technology
Man-made consolidated cellulosic materials, such as fiberboard, can be embossed or molded to have three-dimensional shapes and various design and structural features found in natural wood. Types of useful consolidated cellulosic materials are referred to by the following terms, for example: (a) fiberboards such as hardboard (e.g., low-density hardboard), soft board, and medium-density fiberboard (“MDF”) and (b) chipboards such as particleboard, medium-density particleboard, and oriented strandboard (“OSB”). Composites of these boards are also useful.
Various processes can be used to produce consolidated cellulosic materials, such as those mentioned above. The principal processes for the manufacture of consolidated cellulosic articles include the following: (a) wet felted/wet pressed or “wet” processes, (b) dry felted/dry pressed or “dry” processes, and (c) wet felted/dry pressed or “wet-dry” processes.
Generally in a wet process, cellulosic materials such as fibers (e.g., woody material which is subjected to fiberization to form wood fibers) are blended in a vessel with large amounts of water to form a slurry. The slurry preferably has sufficient water content to suspend a majority of the wood fibers and preferably has a water content of at least ninety percent by weight (“weight percent”) of the wood fibers. The slurry is deposited along with a synthetic resin binder, such as a phenol-formaldehyde resin, onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of about fifty weight percent, based on the dry weight of the fibers. The wet mat is transferred from the pervious support member to a press and consolidated under heat and pressure to form the molded wood composite.
A wet-dry forming process can also be used to produce cellulosic composites. Preferably, a wet-dry process begins by blending cellulosic or wood fiber raw material in a vessel with large amounts of water having a pH of less than seven to form a slurry. This slurry is then blended with the resin binder. As in the wet process described above, the blend is then deposited onto a water-pervious support member, where a large percentage of the water is removed, thereby leaving a wet mat of cellulosic material having a water content of about fifty weight percent, for example. This wet mat is then transferred to an evaporation zone where much of the remaining water is removed by evaporation. The dried mat preferably has a moisture content of less than about thirty weight percent. The dried mat is then transferred to a press and consolidated under heat and pressure to form the wood composite which may be, for example, a flat board, a doorskin, or any other desired shape depending on the intended use of the product.
In a dry process, the cellulosic material is generally conveyed in a gaseous stream or by mechanical means rather than a liquid stream. The cellulosic material may be first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin. The cellulosic material is then randomly formed into a mat by air blowing one or more layers of the resin-coated cellulosic material onto a support member. The mat may optionally be subjected to pre-press drying. The mat, typically having a moisture content of less than about thirty weight percent and preferably less than about ten weight percent, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
During the step of consolidating a cellulosic mat under heat and pressure, a large volume of steam and other gases is typically produced, due to heating of the moisture content of the mat and production of gas during curing of the binder resin, as described for example in Lynch et al., U.S. Pat. No. 5,543,234, the disclosure of which is incorporated herein by reference, and in Reiniger, U.S. Pat. No. 5,093,051. At the completion of the pressing step, opening the press molds typically causes a rapid reduction in pressure in the mold cavity, which sometimes results in the sudden release of water vapor and gases trapped beneath the upper surface of the molded article. This release of gas sometimes causes a portion of the article to “bubble” or “blister” as a portion of the molded surface delaminates from the remaining portion of the article due to the sudden escape of gas from an interior portion of the molded article.
As described in Lynch et al. and Reiniger, for example, forming blisters in a consolidated cellulosic article is considered undesirable, and various techniques are used to avoid blistering. In some mold designs, for example, venting of steam and other gases is accomplished by disposing a gas-pervious material between a back half of the mold cavity and a non-visible side of the product being molded, which allows controlled gas venting without the formation of blisters in the molded article. Other methods of avoiding blistering in consolidated cellulosic products include the use of a vented gas emission control plate, as disclosed in Reiniger.
The thickness, or caliper, of a consolidated cellulosic material is affected by factors such as the amount of material in the cellulosic mat, the temperature and pressure at which the mat is consolidated, and the design of the mold cavity. The desired caliper depends upon the application for which the consolidated cellulosic material is expected to be used. Increasing the caliper of a consolidated cellulosic article may be accomplished by increasing the amount of cellulosic material in the mat, or by laminating two or more consolidated boards together. Both of these methods have the disadvantage of increasing the cost of producing the consolidated cellulosic article, due to the need for increased labor and/or materials to produce the product. Consequently, there is a need for an improved method for producing a consolidated cellulosic article having an increased caliper.